The invention more particularly concerns the fastening of the outer part of the spring, which, in conventional timepiece oscillators, notably with a sprung balance, is generally fastened to a balance spring stud, fastened in turn to a balance cock.
The outer part of the balance spring is generally adhesive bonded in the stud. The bonding position is generally difficult to reproduce, due to shrinkage of the adhesive which is also difficult to reproduce. The quality of the bond is difficult to test once it is made, other than by destructive testing performed on a sample. However, due to the poor reproducibility of the bond, even with a fixed operation, i.e. having tightly controlled parameters, there is no assurance that the sample is representative of the entire batch. The risk is that the balance spring will become detached from the stud once the adhesive has set.
The use of a balance spring comprising an end plate provides a relative improvement in bonding, in that the use of a plate of larger dimensions than the coils of the spring provides a larger bonding contact surface. Indeed, the strength of the adhesive bond is poor when the section and dimensions of the component to be bonded, in this case the balance spring, decrease too much. The plate is required to ensure that a certain amount of contact surface is provided for the adhesive layer, to ensure mechanical strength, which is indispensable since chemical strength alone is not sufficient to securely hold the components to be assembled.
Patent Application DE 2333446 A1 in the name of JUNGHANS GmbH discloses a balance spring, wherein the outer end of the balance spring is placed on a projecting portion of a plate made of plastic material, and held in position by melting the plastic material. The end of the spring can be placed in a slot provided in the projecting portion, whose shoulders may be melted by ultrasonic welding. The fastening is improved by cutting a notch in an edge of the end of the spring in the fastening area. The weld is reversible, and can be removed during repair and conventional adhesive bonding methods remain possible.